CT SCAN equipment.pptx

CT SCAN EQUIPMENTS ARE DESCRIBED IN DETAIL. CONTENT TAKEN FROM MUTIPLE BOOKS AND GENERALS.

1. CT SCAN equipment MR. ROHIT BANSAL ASISTANT PROFESSOR MAMC (AGROHA)

2. CONSOLE ROOM a). Graphic Monitors. b). Keyboard & Mouse. c). Computers. B. EXAMINATION ROOM a). Patient Table. b). Gantry. X-Ray Tube. Generators. Detector Array. Data Acquisition System (DAS). Slip Rings. C. CONTROL ROOM a). AC Plants. b). UPS. CT SCAN SYSTEM COMPONENTS

3. GRAPHIC MONITORS DILOUGE MONITORS: Use for data entry and scan. Now a days being obsolete. VIEWING MONITORS: Used to view reconstructed image in 2D and 3D . Zoom /Planning. Control contrast and brightness. Image analysis technique. INTERACTIVE MONITORS: W hich do works of both dialogue and viewing monitors. CONSOLE ROOM

4. Help to enter patient data quick and quick reference guide. COMPUTERS: Provides the link between CT technologist to other components of the imaging system. It helps to reconstruct an CT image from the microns of mathematical operations required to digitize & reconstruct the raw data. Ct imaging system requires a large computing capacity. The microprocessor and the primary memory are the heart of computer (These determine the reconstruction time). Image can be stored on magnetic tapes, discs, CD and record when required. Keyboard and Mouse

5. The patient couch, or patient table, provides a platform on which the patient lies during the examination . The couch should be strong and rigid to support the weight of the patient. The couch top is usually made of carbon fiber composites because they have low absorption. Gantry: It is a ring that comprised core components of a ct system. It contains a rotating scan frame onto which the x-ray generator, x-ray tube, and other components are mounted. It houses imaging components such as the slip rings, x-ray tube, high-voltage generator, collimators, detectors, and the DAS. Can be tilted +/- 30 degree vertical to the couch. Gantry and couch should be positioned such that the technologist can observe the patient in gantry from operating table. Patient Table

6. Ideally, the radiation source for CT would supply a monochromatic x-ray beam (i.e., one made up of photons all having the same wavelength). With a monochromatic beam, image reconstruction is simpler and more accurate. CT requires larger exposure time at higher kVp then needed for general radiography. In early generation CT scanners were used oil cooled, stationary anode relatively large (2*16mm) focal spot operated at 120 kVp to 140 kVp & 30 mAs heavily filtered x-ray tube (6mm Al equivalent) which help to reduce beam hardening artifact. Now a days CT tubes are heavy duty rotating anode with high atomic capacity & smaller focal spot (0.6-1.2mm) which are air cooled with current value up to 400-800 mA & heat loading capacity of 3.5 MHU. Borosilicate glass envelope has been replaced with a metallic envelope (Alloy of Chromium and Iodine(I)) & Ceramic insulator (Al-Oxide). X-Ray tube

7. Anode disc is made up of compound materials consisting of Tungsten- Rhenium focal spot and graphite base with disc diameter of 200nm compared to 75-160 mm. Anode speed is increased upto 10000-12000 RPM by having liquid bearing technology for smooth rotation of disc. E.g. gadolinium based liquid metal alloy. A Teflon filter is used to match with the contour of body to reduce attenuation unsharpness. (Bow-tie filter). Tube Housing: Material- Borosilicate & metal ceramic. Heat storage capacity- 1.3-1.8 MHU. Oil- Distilled oil.

8. Anode: Diameter- 200nm Weight- 200gm Heat dissipation rate- 1 MHU/MIN Anode angle- 10-12 degree Focal spot- 0.6-1.6 mm Rotation- 10000-12000 RPM/MIN Material- Tungsten+ Rhenium+ Molybdenum+ Graphite Cathode: Material- Tungsten Types- Dual Focus

9. The x-ray beam is collimated at two points, one close to the x-ray tube and the other at the detector(s). Perfect alignment between the two is essential. The collimator at the x-ray tube (Pre patient collimator) is used to shape the x-ray beam and limit patient dose. The collimator at the detector (Post patient collimator) is the sole means of controlling scatter radiation. Each detector has its own collimator. The collimators also regulate the thickness of the tomographic slice (i.e., the voxel length). Most scanners have collimators that can be varied to produce different slice thicknesses, typically from 1 to 10 or 15 mm. Choice of slice thickness is usually limited to one of several predetermined values, with choices of 1, 2, 5, or 10 mm being typical examples. Pixel size is determined by the computer program and not by the collimator. Collimators

10. Detectors Detectors are the device used to measure the amount of electromagnetic radiation emitted by the object. Detectors detect the X-Rays passing through the patient body. Detector collects information regarding the degree to which each anatomical structure attenuate the beam during a scan. The requirements of CT detectors are:- Should be small with good resolution. Should have high detection efficiency. Should have fast response. Negligible after glow effect. Stable noise free response.

11. Two types of detectors are in use nowadays. (a) Xenon gas detector. (b) Solid state detector. Types of Detectors

12. Gas filled ionization detector consist of the Xenon gas and krypton gas chambers (1000 chambers) under high pressure around 25 to 30 atm to increase the number of gas molecules available for ionization which improves their x-ray detection efficiency. These chambers individually separated by tungsten plates known as metal septal enclosed by a thick ceramic substrate material. Each chambers (1000) share the same gas volume to minimizing variation in sensitivity between the chambers. As x-rays interact with the xenon gas atoms, ionization takes place then the electric field between the plates cause the charge ions to move towards the electrodes where the electronic charge is collected as electric current. Then the electric signal is amplified and then digitized which numerical value is directly proportional to the x-ray intensity striking the detectors. Gas Ionization Detector

13. Used mainly in rotate-rotate system because use of long linear structures. Chambers size- wide:1-2 mm, height: 10 mm, depth: 10 cm. Gas- Mixture of Xenon and Krypton. Anode- Made up of copper. Cathode- Made up of Tantalum (Ta). Pressure- 25-30 atm. No afterglow. Very fast Response.

14. Cheap and easily available. It has constant sensitivity at long and short term. Provides in-built collimation. Rapid response with no afterglow effect. Have better thermal stability. Advantages

15. Quantum detection efficiency is less than 50 which is about 50% of solid state detectors. The xenon has to be maintain high pressure to increase detection efficiency and higher response to exposure. Must be set in a fixed positioned orientation to the x-ray source. Reception of x-rays from a moving source is not efficient. For the above two reason gas detectors are not suitable for fourth generation and helical scanners. Disadvantages

16. It is composed of a “scintillator” coupled tightly with a ‘photodiode’ or ‘PM-tubes’. The ‘Scintillator’ emits visible light when it is stuck with x-rays. The light emitted by the Scintillator reaches either ‘PM-tubes’ or ‘photodiodes’ which is an electronic device that converts light photons into an electrical signal. Now a days commonly Scintillation coupled with solid state photodiodes. Scintillation material used with photodiodes are ceramic material made of high purity ‘rare earth oxides’ based on dopped rare earth compounds as ‘Yettrium & Gadolinium oxysulphides (Gd2O2S) ultra fast ceramic (UFC) detectors. In early CT-scanners (1 st and 2 nd generation) Sodium Iodide ( NaI ) were used with Thallium ( Nai:Ti ) crystals coupled with PM-Tubes as x-ray detector. Solid State (Scintillation Crystal Detector)

17. The PM Tubes could not provide packing density that was necessary with fan shaped beam geometry and required power supply for each PM Tube makes bulky detector assembly, so because of poor packing density higher afterglow problems are seen that’s why it was quickly replaced by silicon photodiodes. Detector element converts the absorbed x-rays into light. The light is then detected by si -photodiodes. The resulting electric current is then amplified and convert into a digital signal.

18. NaI:Ti Used in 1 st generation CT scan, 100% efficient in diagnostic range. Not suitable for rapid sequential exposure. CsI:Ti Crystal structure variation affect the efficiency. CdWO4 Low afterglow but toxic. BGO [Bismuth Germinate Oxide] (Bi4Ge3O4) Difficult in matching. Modern Detectors Ceramic rare earth phosphors like Gd2O2S, Y2O2GdO2:Eu.

19. Photodiodes are smaller in size, greater stability and cheap. Do not require a power supply and are equally efficient. Solid state detectors have better x-ray absorption efficiency then xenon gas detectors. Increase efficiency can result in less x-ray tube loading (reduce image noise and reduce patient dose). The top surface of solid state detectors are essentially flat and therefore it is capable of x-ray detection over a wide range of angels unlike the xenon detector. Advantages

20. Relatively long afterglow. Stable output signals depends upon a very high voltage supply. Multi-detector uniformility of response is difficult to maintain. It may be less stable in long term, they require time to time calibration. Disadvantages

21. Good sensitivity (detection efficiency is good). Fast response time. Stable (could not easily affected by varying temperature, humidity, voltage etc.) High conversion efficiency. Small afterglow. Cost effective. Image quality must be good. High packing density. Wide dynamic range (able to detect wide number of x-ray) Properties of a Ideal Detector

22. It generates high voltage potential between cathode and anode of the x-ray tube. All CT-Imaging systems operates on high frequency power. A high frequency generator is small in size, so it can be mounted on the rotating gantry. kVp selection (Tube Potential)- 80-140 mAs selection (Tube current)- 20-500 All multislice spiral CT scanner operates on high frequency power. CT scanner uses three phase power for efficient production of x-rays. In CT scanner generators low voltage low frequency is converted into high voltage and high frequency (500-2500 Hz) direct current of almost constant potential is supplied to the x-ray tube. High Voltage Generator

23. Slip rings are electromechanical device that conduct electricity and electrical signal through rings and brushes from a rotating surface on to fixed surface. One surface is smooth ring (stationary) and other is a ring with brushes (rotating) that sweep the smooth ring. Spiral ct is made possible by the use of slip ring technology, which allows the gantry to rotate without interruption. Composite brushes are made up to conductive materials (e.g. silver graphite alloy)are used. The brushes have to be replaced every year for preventive maintenance. Slip Rings

24. Data acquisition is a process of sampling signals that means real world physical signals converting into digital numerical values that can be manipulated by computers. DAS typically convert analog waveform into digital value for easy processing. PROCESS OF DAS: Physical signal: input of DAS can be represented in digital form are like temperature, pressure, light, force etc. Transducer/Sensor: convert physical signal into electrical waveform. (Noisy) DAS (Data Acquisition System)

25. Signal Conditioning: filter and amplifies noisy electrical waveforms. ADC: Convert analog signal into digital signal. Pre- Processing: Image reconstruction (Back projection, Iterative reconstruction, Filter back projection). DAC: convert digital signal into analog signal. (Video/Picture) Image Display: image displayed on computer. Post Processing: 3D, angiographic image etc. Image Archiving: Image Archive through PACS.

26. Power loss and power surges are good reasons to have back-up power, like a UPS, hooked up to your medical equipment. Say a power loss occurs in your facility, the UPS (uninterruptible power supply) seamlessly kicks in to keep the equipment from shutting down for a short period of time, so that you are able to save what you are working on and safely shut the system down. AC PLANTS: AC plants helps in controlling the temperature, humidity in the CT Scan room. UPS